on the website I see where you can log-in and log-out, but I can't find this on the iphone app? Where do you log out?

In the official Toodledo app, there is no "log out". You can Unlink the app from an account, or you can change the Email login to change the account.

But ... you will end up with all tasks in both accounts (on the iPhone app AND on the website) if you don't "Purge Data" before linking to the second account. Actually, I'm not sure about this, but I'm not going to try it. ;-)

Probably the easiest way to accomplish this is to use two different iPhone apps, each dedicated to one account. Appigo's Todo app is a good option. You could also dedicate the mobile site for one account through iPhone's Safari.

The iPhone app is designed for one user. I would not recommend regularly, purging data and logging in under a new user. This will cause problems with syncing and duplication of data. I recommend what JPR said.